Organic crystalline films for optical applications and related methods of fabrication

ABSTRACT

A method for forming an optical device includes the steps of providing a first plate having a first face defining a recess, filling the recess with a material which can be crystallized, and covering the first face and the recess with a second plate having a second face, so that the second face is in contact with the first face and the material in the recess is completely enclosed by the first and second plates. The material in the recess is thereby protected from chemical and mechanical damage, as well as evaporation. In addition, the plates can be transparent, allowing the material in the recess to be visually monitored. A grown crystalline film packed in the cell can be used as a non-liner and/or electro-optical device.

FIELD OF THE INVENTION

The present invention relates to organic films, and more particularly toorganic crystalline films.

BACKGROUND OF THE INVENTION

Films of organic crystalline materials can be used in various opticalapplications. For example, optical waveguides made from organic materialinclude slab waveguides in which light is confined in only onedimension, and 2-D channels of size 1 cm×1 μm×1 μm. In addition,cylindrical geometry devices filled inside and outside by a non-linearmaterial and surrounded by an electrical field have been reported.

The growth of thin single crystals is discussed, for example in thereference by K. M. M. Kruse entitled "Apparatus and Method For TheGrowing Of Single Crystal Specimens Of Organic Substances For InfraredSpectroscopic Investigation," J. of Physics E: Scientific Instr., vol.3, pp. 609-14, 1970, Great Britain. The crystals are grown from the meltbetween NaCl (or KBr) windows held at a distance of about 25 μm apart bymeans of a thin spacer (Polyester foil). A temperature gradient ismaintained first along a capillary and then along the largest dimensionon the NaCl windows, resulting in a clear definition of thecrystallizing zone. The crystallizing zone is slowly raised by loweringthe cell along the vertical temperature gradient within a heated column.

A second reference by Pech et al. studies the growth of solidbenzophenone from its own melt contained in a crucible submitted to aunidirectional temperature gradient. Pech et al., "A New Technique ForDetermining The Kinetics Of Crystal Growth From The Melt," J. of CrystalGrowth, vol. 43, no. 1, 123-25, 1978. In this reference, a sample wasplaced in a 1×10×15 mm³ crucible made of glass plates. Thermal boundaryconditions were imposed by two heating blocks fixed to the extremitiesof the crucible and connected to thermostats. The crucible is fixed tothe substage of a microscope so that the moving liquid-solid interfacecan be maintained in coincidence with the cross-hair reticule of theobjective of the microscope.

Another method for producing a crystal film is discussed in the Hattoriet al. patent entitled "Method For Producing Organic Crystal Film." U.S.Pat. No. 5,385,116. In this patent, a crystal film on an organiccompound is produced from a molten liquid between a pair of substrates.At least one of the pair of substrates has on a part of a surfacethereof a three-dimensional geometrical structure capable of controllingthe direction of crystal growth of the organic compound. The other partof the surface having the three-dimensional geometrical structure issmooth.

U.S. Pat. No. 4,793,893 to Thakur et al. entitled "Methods For ThePreparation Of Thin Large-Area Single Crystals Of Diacetylenes AndPolydiacetylenes" discusses a method for preparing thin large-areasingle crystals of diacetylene monomer. This method involves forming aliquid layer containing pure diacetylene monomer between two opposedsurfaces; applying pressure to the liquid layer disposed between the twoopposed surfaces; and crystallizing the liquid layer disposed betweenthe two opposed surfaces while by evaporation the liquid layer is keptunder constant pressure to form a thin large-area single crystal of purediacetylene monomer. This patent also discusses a method for preparing athin large-area single crystal of pure diacetylene monomer.

Notwithstanding the above mentioned references, there continues to exista need in the art for improved structures including organic crystals andrelated methods for forming organic crystals.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide improved methods and structures for organiccrystalline films.

This and other objects, features and advantages according to the presentinvention are achieved by providing a first plate having a first face.The first face defines a recess which is filled with a material whichcan be crystallized, and covered with a second plate having a secondface. Accordingly, the second face is in contact with the first face andthe material in the recess is completely enclosed by the first andsecond plates. The material within the recess is thereby protected bythe plates from chemical and mechanical damage as well as evaporation.

The material in the recess can be crystallized, and the step ofcrystallization can include the steps of heating the material above amelting temperature, and cooling the material to obtain a homogeneousdistribution. The crystallization step can be used to produce a singlecrystal film of the material in the recess. The single crystal film canprovide optical properties that are superior to those of polycrystallinefilms. In addition, the material can be an organic compound which hasnon-linear optical properties.

The plates used to contain the material are preferably transparentallowing the material to be visually monitored during thecrystallization step. For example, the plates can be formed of fusedquartz, which is preferably optically polished to reduce the formationof defects during the crystallization step.

Furthermore, the first face of the first plate preferably defines agroove surrounding the recess. This groove can then be used to contain aportion of the material. For example, if the recess is initiallyoverfilled, or if the material overflows from the recess as a result ofthermal expansion, the excess can be contained by the groove.Accordingly, any excess material will be prevented from separating thetwo plates.

In another embodiment of the present invention, an optical deviceincludes a first plate having a first face which defines a recess, acrystalline material in the recess, and a second plate having a secondface wherein the second face covers the first face and crystallinematerial in the recess. The first face of the first plate may furtherdefine a groove surrounding the recess, and a portion of the crystallinematerial may be contained in the groove surrounding the recess.

The first and second plates are preferably first and second transparentplates such as first and second fused quartz plates. In addition, thesecond face of the second plate and a surface of the recess opposite thesecond plate are preferably optically polished. The crystalline materialmay be an organic compound, and it is preferably a single crystal film.

The methods and structures of the present invention provide an organicsingle crystal thin film which can be used in non-linear opticalapplications. The structure can be reproducibly fabricated withrelatively little expense. In addition, the thin film in the recess isprotected from chemical and mechanical damage, as well as evaporation,by the two plates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 show cross-sectional views of various steps in the fabricationof a thin crystalline film according to a method of the presentinvention.

FIG. 5 shows a top view of a plate including a recess and a groveaccording to FIG. 1.

FIG. 6 shows an experimental set up according to the present invention.

FIG. 7 shows a crystal in polarized light according to the presentinvention.

FIG. 8 shows a set up for measuring the second harmonic generation ofcrystalline films according to the present invention.

FIG. 9 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 1 μm. E is perpendicular to n.sub.τ. φ_(max) =70°, θ_(max) =5°.

FIG. 10 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 1 μm. E is parallel to n.sub.τ. φ_(max) =71°, θ_(max) =6°.

FIG. 11 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 2 μm. E is perpendicular to n.sub.τ. φ_(max) =11°, θ_(max) =38°.

FIG. 12 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 2 μm. E is parallel to n.sub.τ. φ_(max) =13°, θ_(max) =52°.

FIG. 13 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 5 μm. E is perpendicular to n.sub.τ. φ_(max) =66°, θ_(max) =20°.

FIG. 14 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 5 μm. E is parallel to n.sub.τ. φ_(max) =71°, θ_(max) =5°.

FIG. 15 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 10 μm. E is perpendicular to n.sub.τ. φ_(max) =9°, θ_(max) =61°.

FIG. 16 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. Crystal thicknessis 10 μm. E is parallel to n.sub.τ. φ_(max) =19°, θ_(max) =40°.

FIG. 17 shows the intensity of the second harmonic versus rotation anglealong b-axis (parallel to n.sub.α). Crystal thickness=10 μm.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the present invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art. Inthe drawings, the thickness of the layers and regions are exaggeratedfor clarity. Like numbers refer to like elements throughout and primenotation is used to indicate like elements in various embodiments.

An organic crystalline film, according to the present invention, can befabricated by a method illustrated in FIGS. 1-4 and 5. In this method, athin crystalline film is grown between two plates. As shown in FIGS. 1and 5, a first plate 20 has a first face 22, and the first face definesa recess 24. The recess 24 can be on the order of 10 mm in diameter, andanywhere from 0.5 to 500 micrometers deep. The first face may alsodefine a groove 26. As shown in FIG. 5, the recess and groove are bothcircular. As will be understood by those having skill in the art,however, the recess and groove may have other shapes such as oval,oblong, square, etc.

As shown in FIG. 2, the recess is filled with a material 28 such asmeta-nitroaniline (m-NA), 2-methyl-4-nitroaniline (MNA),(-)2-(alpha-methylbenzylamino)-5-nitropyridine (MBANP),N-(4-nitrophenyl)-(L)prolinol (NPP), benzyl, or other compounds known tothose having skill in the art which can be crystallized by heating thematerial to obtain a homogeneous distribution and then cooling thematerial. Other organic materials are disclosed for example in thefollowing references: K. M. M. Kruse, "Apparatus and Method for theGrowing of Single Crystal Specimens of Organic Substances for InfraredSpectroscopic Investigation," J. of Physics E: Scientific Instr., vol.3, pp. 609-14, 1970 (Great Britain); Pech et al., "A New Technique forDetermining the Kinetics of Crystal from the Melt," J. of CrystalGrowth, vol. 43, no. 1, pp. 123-25, (1978); U.S. Pat. No. 5,385,116 toHattori et al. entitled "Method for Producing Organic Crystal Film"; andU.S. Pat. No. 4,793,893 to Thakur et al. entitled "Methods for thePreparation of Thin Large-Area Single Crystals of Polydiacetylenes."Each of these references is hereby incorporated herein in its entiretyby reference.

The first face 22 of the first plate 20 and the recess 24 are thencovered by a second plate 30 so that the second face 32 of the secondplate 30 is in contact with the first face 22 of the first plate 22.Accordingly, the second plate 30 acts as a lid, enclosing the material28 within the recess 24 as shown in FIG. 3. The material 28 ispreferably a thin polycrystalline film at this point. The material canbe heated above its melting point and then cooled in order to form athin polycrystalline film with a homogeneous distribution. As shown,this film may contain a plurality of crystal grains, including thesingle crystal grain 28', divided by grain boundaries.

The plates are both preferably transparent, so that the material 28 canbe visually monitored during the various steps of the fabrication of thesingle crystal film. For example, the two plates can be fused quartzplates. In addition, the plates are optically polished allowing the twoplates to fit closely together and also to avoid the formation of seedsduring the crystallization step.

The material is then crystallized to form a single crystal thin film 28aof the material which is completely enclosed within in the recess 24between the two plates, as shown in FIG. 4. The material 28 can becrystallized by heating above the melting point of the material (112degrees C. for m-NA) and then slowly cooling the material so that asingle crystal film forms from a single crystal grain. For example, thematerial 28 can be completely melted except for the single crystal grain28'. When the material is slowly cooled, a single crystal thin film 28agrows from the single crystal grain 28'.

The groove 26 surrounding the recess 24 can be used to contain an excessportion 28b of the material which overflows from the recess 24. Forexample, the overflow may be caused by initially overfilling the recessor as a result of heating the material thereby resulting in thermalexpansion of the material. By providing a groove 26 where excessportions 28b of the material can accumulate, the excess material isprevented from remaining between the faces of the two plates.Accordingly, the two plates can be maintained in direct contact withoutbeing separated by excess portions of the material. The faces of the twoplates form a tight seal protecting the material within the recess fromchemical or mechanical damage as well as from evaporation.

The two plate structure of FIGS. 3 and 4 defines a cell 36 which can beincorporated in the experimental setup of FIG. 6. The cell 36 ispositioned in a heat exchange chamber 38 in which a desired temperatureis achieved by pumping a liquid heat carrier from a high temperaturecirculator 40 with a temperature controller. The heat exchange chambercan be brass, and the temperature circulator can be a commerciallyavailable temperature circulator such as a Neslab EX-250HT hightemperature controller. The liquid heat carrier can be distilled waterfor growing crystals with melting points below 100 degrees C., or acommercially available liquid such as Syntherm 800 liquid having aboiling point higher than that of water for growing crystals withmelting points above 100 degrees C. The heat exchange chamber 38 and thecell 36 are placed on the stage 41 of a polarizing microscope 42 forobservation and control of single crystal growth. The stage preferablyhas three degrees of freedom of motion, allowing observation ofdifferent regions of the crystal during crystal growth.

In one embodiment of a method according to the present invention, thematerial in the recess formed by the plates of the cell is heated andthen cooled to obtain a homogeneous distribution as shown in FIG. 3. Thecell 36 is then placed in the heat exchange chamber 38 as shown in FIG.6, and a single crystal grain 28' having a desired crystallographicorientation is visually located using the microscope 42. The temperatureof the cell 36 is then increased by pumping the liquid heat carrierthrough the heat exchange chamber 38 in order to melt the material 28.The single crystal grain 28', however, is maintained in its crystallineform by specifically cooling that grain. This localized cooling can beachieved by using a microjet 44 to blow cool air onto the cell 36 at thelocation of the single crystal grain. The melted material is then slowlycooled so that the single crystal acts as a seed. Accordingly, thematerial forms a single crystal film having the same crystallographicorientation as the single crystal grain. While the use of a microjet isdisclosed as a means for providing a single crystal grain as a seed forcrystal growth, other techniques for providing a seed will be known tothose having skill in the art.

The thus grown crystal is packed between the two plates preventing itsevaporation and mechanical damage. In addition, the microscope allowsthe crystallization method to be performed manually. Alternately, themethod can be automated by using a CCD camera and a computer as will beunderstood by those having skill in the art.

Crystals have developed sliding systems. The highest rate of crystalgrowth takes place along the highest density of molecular packing (HDMP)in the cleavage plane. In most crystals having symmetries higher thantriclinic, the cleavage plane is typically parallel to thecrystallographic plane formed by the smallest parameters, and thedirection of growth is parallel to the shortest axis of the latticecell. In the case of m-NA, the cleavage plane will be the (010) plane,and the direction of growth will be the 001! direction.

When the temperature is decreased from near the melting point of thematerial, to the ambient temperature, the crystal packed in the recessbetween the two plates of the cell, experiences strain due to thedifference in coefficients of temperature expansion of the crystal andthe plates. If the crystal has a thickness greater than 3 micrometers,microscopic cracks may form. The direction of these cracks is collinearwith the highest density of molecular packing, and the density of thecracks may be correlated with the thickness of the crystal. In somecrystals with thicknesses greater than 10 micrometers, an additionalsystem of more fine and less rectilinear cracks may form in thedirection perpendicular to the highest density of molecular packing.Crystalline films with a thickness of less than 1 μm have been found tohave no cracks due to the higher withstanding strain.

The material 28 used to form the thin single crystal film 28a can be anorganic material such as m-NA. Orthorhombic m-NA (NO₂)C₆ H₄ (NH₂)!belongs to the space group Pbc2₁ (point group mm 2), with Z=4, andlattice parameters a=6.501 Angstroms, b=19.33 Angstroms, and c=5.082Angstroms. Various properties of m-NA are given below in Table 1.

                  TABLE 1                                                         ______________________________________                                        Name         3-nitroaniline                                                   (Abbreviation)                                                                             (m-NA)                                                           and          O.sub.2 NC.sub.6 H.sub.4 NH.sub.2                                Chemical Formula                                                              m.p. °C.                                                                            112                                                              Symmetry     Pbc2.sub.1, C.sub.2v.sup.5                                       Parameters of                                                                              a = 6.501   b = 19.33 c = 5.082                                  cell, Å                                                                   Gliding System                                                                             (010),       001!                                                Refraction indices*                                                           1064 nm      n.sub.1 = 1.74                                                                            n.sub.2 = 1.71                                                                          n.sub.3 = 1.65                              532 nm      n.sub.1 = 1.81                                                                            n.sub.2 = 1.79                                                                          n.sub.3 = 1.72                             ______________________________________                                    

The strongest intermolecular interaction takes place in the (010) planealong the direction of the c-axis. Besides van der Waals interactions,hydrogen bonding (O . . . H-N and N . . . H-N) contributes to thecrystal energy. Both types of interactions are responsible for adouble-layer structure which is perpendicular to the crystallographic010! axis. Observations reveal that m-NA is unstable and oxidizes at itsmelting point. Recrystallization of the m-NA by the methods of thepresent invention does not significantly shift the edge of absorption.At the first stage of growth from a seed, the crystal had an arrow likeshape with a longer side aligned perpendicular to the larger index ofrefraction in the plane of the crystal refraction index. For the m-NAcrystal, this is also the largest refraction index of the crystal.

By maintaining a single crystal grain of the m-NA as a crystal growthseed by localized cooling, such as by a microjet as discussed above,while melting the rest of the m-NA and then slowly cooling the m-NA, theseed will orient along the temperature gradient. As the crystal seedgrows, the formation of spikes of crystal growth, which can result intwin defects, is avoided by slowly lowering the rate of growth whileobserving the crystal growth. If defects are observed, the temperaturecan be raised to remelt the defect and then lowered to continue crystalgrowth. Despite the high asymmetry of growth rates, large-area highquality crystalline films can be grown. FIG. 7 shows a photograph of am-NA crystalline film in cross-polarized light. This film is about 10microns thick.

According to various studies, m-NA is a negative biaxial crystal.Refraction indices of m-NA for wavelengths of 1,064 nm and its harmonic0.532 nm are given in Table 1 above. m-NA belongs to class 13 ofHobden's classification, and phase matching conditions are observed inthe cone of directions about the optic axis only for processes involvingfundamental waves of parallel polarization (type I).

FIG. 8 shows a schematic diagram of an experimental setup for secondharmonic generation measurements. A 1064 nm wavelength laser 50 such asa Quantel Q-switched Nd:YAG laser (1064 nm) with a 10 HZ pulserepetition rate can be used as a fundamental source. The intensity ofthe fundamental beam can be on the order of 100 mW/cm². The crystal 52can be preliminarily oriented using the polarizing microscope(illustrated in FIG. 6), and placed onto a goniometer head with itsplane perpendicular to the incident beam 54. The polarization plane ofthe incident beam can be rotated by a double-Fresnel rhomb 56, and seteither parallel or perpendicular to the c-axis of the crystal 52. Theoutput radiation beam 58 from the crystal can be filtered through filter60 to eliminate the fundamental 1,064 μm radiation. The generated secondharmonic can be detected by a detecting system 62 which may include alens to focus the second harmonic and a diode detector such as aHamamatsu S1337-BQ series diode detector. The averaged signal generatedby the detecting system 62 can be observed and measured by anoscilloscope such as a Tektronix model 7704 oscilloscope. Theexperimental setup can also include a polarizer 64 to analyze thegenerated second harmonic.

A crystal can be initially positioned on the stage of a microscope withthe n.sub.τ axis horizontal, and the n.sub.β axis vertical. Rotating thecrystal along the vertical axis, the orientation resulting in themaximum intensity of the second harmonic can be found thus defining thedirection of matching conditions. The intensity of the second harmoniccan be measured versus azimuthal rotation angle (along the n.sub.β axis)and tilting angle (around the n.sub.τ axis).

The intensity of the second harmonic can be determined mathematically bythe following equation:

    I.sup.2ω ={2ω.sup.2 / .sub.o C.sup.3 (n.sup.ω).sup.2 n.sup.2ω !}d.sup.2 L.sup.2 (I.sup.ω).sup.2 Sinc.sup.2 (ΔKL/2)

In this equation, L is the thickness of the film, d is the effectivenon-linear susceptibility, sinc(x)=(sinx)/x, and x=(ΔKL/2) is the phasemismatch between the fundamental and second harmonic waves.

In order to measure the intensity of the second harmonic, the 1064 nmradiation of the laser 50 of FIG. 8 is vertically polarized, and thepower of the laser is reduced to a value on the order of 2.5 mW toprevent damage to the crystal. The polarization and intensity of thesecond harmonic generated by thin crystalline films of m-NA versus theangle of rotation along the vertical and horizontal axes can bemeasured.

These measurements were made and the resulting data is graphicallyillustrated in FIGS. 7-17. In the graphs of FIGS. 9-16, the horizontalaxis represents the angle in units of degrees, and the vertical axisrepresents the intensity of the second harmonic in arbitrary units. InFIG. 17, the horizontal axis represents the angle of rotation along theb-axis in degrees, and the vertical axis represents the intensity of thesecond harmonic in arbitrary units.

FIG. 9 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. The crystalthickness is 1 μm. E is perpendicular to n.sub.τ, φ_(max) =70°, andθ_(max) =50°. FIG. 10 shows the intensity of the second harmonic versusazimuthal (φ, triangles) and tilting (θ, circles) rotation angles. Thecrystal thickness is 1 μm. E is parallel to n.sub.τ, φ_(max) =71°, andθ_(max) =6°. FIG. 11 shows the intensity of the second harmonic versusazimuthal (φ, triangles) and tilting (θ, circles) rotation angles. Thecrystal thickness is 2 μm. E is perpendicular to n.sub.τ, φ_(max) =11°,and θ_(max) =38°. FIG. 12 shows the intensity of the second harmonicversus azimuthal (φ, triangles) and tilting (θ, circles) rotationangles. The crystal thickness is 2 μm. E is parallel to n.sub.τ, φ_(max)=13°, and θ_(max) =52°.

FIG. 13 shows the intensity of the second harmonic versus azimuthal (φ,triangles) and tilting (θ, circles) rotation angles. The crystalthickness is 5 μm. E is perpendicular to n.sub.τ, φ_(max) =66°, andθ_(max) =20°. FIG. 14 shows the intensity of the second harmonic versusazimuthal (φ, triangles) and tilting (θ, circles) rotation angles. Thecrystal thickness is 5 μm. E is parallel to n.sub.τ, φ_(max) =71°, andθ_(max) =5°. FIG. 15 shows the intensity of the second harmonic versusazimuthal (φ, triangles) and tilting (θ, circles) rotation angles. Thecrystal thickness is 10 μm. E is perpendicular to n.sub.τ, φ_(max) =9°,and θ_(max) =61°. FIG. 16 shows the intensity of the second harmonicversus azimuthal (φ, triangles) and tilting (θ, circles) rotationangles. The crystal thickness is 10 μm. E is parallel to n.sub.τ,φ_(max) =19°, and θ_(max) =40°. FIG. 17 shows the intensity of thesecond harmonic versus rotation angle along b-axis (parallel ton.sub.α). The crystal thickness is 10 μm.

Applicant notices a discrepancy between φ_(max) and θ_(max) values fordifferent crystal thicknesses. These values almost coincide for the 1 μmand 5 μm crystal thicknesses, whereas for a thicker 10 μm crystal, themaximum of the second harmonic intensity is achieved along a directionalmost in the n.sub.τ n.sub.β -plane. Applicant theorizes that for thincrystals (1 μm and 5 μm thick) whose optical path is less than thecoherence length (at 1,064 μm varying in the range from I₃₂ =16 to I₃₃=3.9 μm), the mechanism of the second harmonic generation is differentfrom that in thicker crystals. Observations reveal that in thin crystalsφ_(max) and θ_(max) values correspond to the light propagating in a conewith an angle X satisfying the Cerenkov condition:

    Cos χ=n.sub.eff /n.sub.c

In this equation, n_(eff) and n_(c), are the crystal and cell materialindices of refraction, respectively. The φ_(max) and θ_(max) values forthe thick crystal roughly correspond to those estimated for m-NA crystalfrom the Fresnel equation.

Zero along the horizontal axis indicates the direction along which themaximum of the second harmonic intensity was observed (the matchingcondition in the n.sub.τ n.sub.α -plane). This result is significant indefining the orientation of the crystal for the most efficientnon-linear performance (i.e. the phase matching direction is to belooked for in the plane perpendicular to the cleavage plane).

The method of the present invention provides a number of advantages.Single crystal films of m-NA with low concentrations of defects can beformed having a surface area of up to 1 cm² and thicknesses ranging from0.5 to 500 microns. By forming crystals with a thickness of less than 3microns, defects can be reduced, and in particular, the concentration ofdislocations can be reduced. These single crystal films have a highernon-linear efficiency performance than that of polycrystalline films. Inaddition, the geometry of these single crystal films allows eitherdiffraction and/or plane mode elements to be formed. The geometry ofthese films also allows the Cerenkov conditions for second harmonicgeneration to be achieved.

The use of the two plate structure including a recess and a groove,protects the single crystal film in the recess from mechanical andchemical damage. The crystal orientation of the film can be readilydetermined using interference techniques. Furthermore, the structureincluding the single crystal film in a recess between two platescombines the advantages of a crystalline structure, such as the phasematching condition which increases the efficiency of second harmonicgeneration, with that of optical waveguides. Accordingly, the number ofmodes generated can be limited for optical switching applications suchas for optical computers and imaging.

Changes in temperature of films less than 3 microns thick can lead todeformation without destroying the film or causing defects. Thesedeformations can change the refractive indices. In particular, lowerelectronic transitions, such as exciton bands, can be changed as aresult of changing the temperature, thereby changing the opticalproperties of the crystal. This property effectively widens the opticalapplications of these crystalline films. The method of forming crystalsof the present invention can further be used to screen organic compoundsother than m-NA for non-linear optical applications.

Many modifications and other embodiments of the invention will come toone skilled in the art having the benefit of the teachings presented inthe foregoing descriptions and the associated drawings. Therefore, it isto be understood that the invention is not to be limited to the specificembodiments disclosed, and that modifications are intended to beincluded within the scope of the appended claims.

That which is claimed:
 1. A method for forming an optical device, saidmethod comprising the steps of:providing a first plate having a firstface, said first face defining a recess; filling said recess with amaterial which can be crystallized; and covering said first face andsaid recess with a second plate having a second face, so that saidsecond face is in contact with said first face and said material in saidrecess is completely enclosed by said first and second plates.
 2. Amethod according to claim 1 further comprising the step of crystallizingsaid material in said recess.
 3. A method according to claim 2 whereinsaid crystallizing step comprises the steps of:heating said materialabove a melting temperature; and cooling said material to obtain ahomogeneous distribution.
 4. A method according to claim 2 wherein saidcrystallizing step comprises crystallizing said material to form asingle crystal film of said material in said recess.
 5. A methodaccording to claim 1 wherein said material comprises an organiccompound.
 6. A method according to claim 1 wherein said first and secondplates comprise first and second transparent plates.
 7. A methodaccording to claim 6 wherein said first and second transparent platescomprise first and second fused quartz plates.
 8. A method according toclaim 1 wherein said providing step comprises providing said first platehaving said first face, said first face further defining a groovesurrounding said recess.
 9. A method according to claim 8 furthercomprising the step of containing a portion of said material in saidgroove.
 10. A method according to claim 1 wherein at least one of saidplates comprises a transparent plate, said method further comprising thestep of providing a light source for illuminating said material.
 11. Amethod according to claim 10 further comprising the step of providing anoptical detector for detecting illumination of said material.
 12. Amethod of forming an optical device, said method comprising the stepsof:providing a first plate having a first face, said first face defininga recess; providing a material which can be crystallized in said recessof said first plate; and providing a second plate on said materialopposite said first plate wherein said first and second plates comprisefirst and second transparent plates.
 13. A method according to claim12:wherein said step of providing said material comprises filling saidrecess with said material; and wherein said step of providing saidsecond plate comprises covering said recess so that said material isenclosed therein.
 14. A method according to claim 13 wherein said stepof providing said first plate comprises providing a first plate having afirst face, wherein said first face further defines a groove surroundingsaid recess.
 15. A method according to claim 14 further comprising thestep of containing a portion of said material in said groove.